OFfSHORE OF

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FATIGUE CRACK GROWTH CHARACTERISTICS OF OFfSHORE STRUCTURAL STEEL IN MARINE

ENVIRONMENT

By

@BIBHAS BHATTACHARYA,B. Tech.

A Thesis submitted to the School of Graduate Studies ill partial fulfillmcnt of the requircments for the degreeof Master of

Engineering

Faculty of Engineering and Applied Science Memorial Universityof Newfoundland

April,1993

St. John's Newfoundland Canada

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1+'

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ABSTRACT

Corrosionfi\tiglll~ofi\nll'lali.~a lIlor£,~l'l'l'n'[,miloffi\ti~1H'1'11"n"I1I<'ua in the prCSClI<:.e of a corrosive environmenl likc sP,(wah'r or slimI;i~~, TIll' cnh ..l.nccd fatigtlc crack growth ril.teh;l.~bccrl il.Urillllkd ill ['arL I"till';"""Ii.

dissolution (oxidatiou of metal) (','t'IC crack lip and Lohydrugl~1l(·mhriLtI.··

menlo Numerous studies hav(' producedawillt~varietyor n':lults n'l;"nliull:

the cITed of anotlic ,lissolllt;on ami r,(Lhu.lic l'roll,(·tiuutilltilemrretSi"ll fa tigue rate of high str('ngth steel. Anodic dissolution ("all citlll'rf;I.lISCil.lI enhanced crack growth rate or a crack lip blunting("ansin~a 1Il1wh ,lday",l cra.ck initiation or (:Y('ll a crack arrest. TllCrc is,how"v,~r,:;<'111" ... ms'·lIs"s about all optimum range of cathodic protl'diull potlmt;al(-x:mlltv SeE), around whichthe offshoresL,~~lsa.chieve llw IlIi1.Xillllllll rorrosiollfa1.i~l1l'lifo:.

A more positive potl·lIliil.lfails to slop 1.11(' ;1.'1l"lir dissnlllUulI. /\Ilion'1I"J~a' tive potential ca.uses a higher crack growtb rale, (1I10st]ln~siblyhy 1!,"lU'rali"u of hydrogcn and hydrogen cmbrittlemcnt).Micro-orgi\ni~rns,l':;l'ecially Sui·

fatc Reducing Bacteria have been known to have ('lIlmIlCI:,1 mdal "..rr".~i"n fatigue. There are mainly twomcehanisillsrcspollsib)l~for SII1;1ICllh;I.Il""I1It~nl.

Duringthe respirationprocesshydrogenisconsulIled from l!leliyst(~mwllidl causes a cathodic depolarisation. Ilydrogen sulphide is pmdur.,:d ilt L111' "1l11 ofthe respiration cycle, which is known for its conlrilmlioll Lo hy,lrul;"n embrittlemcnl.

Il is important to understand mort: ahollt ther.orrosiOIlfittigllt~plll'lI"lIl'

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'~Ila(If"rfsh(Jr,~sLrlldural sh:cl in1\marine etlvironillent.[nthe present study

<:urrosion ratigllc bchavit· or CSA G 40.21 M .150WTstccl is investigaled.

T,~sts Wf~r<' m/lfII/,;t(~1ill ,tic,Sl'itlWtlCJ aud sCitwalcr with calhodic protN;tiun.

COlllpad Ttllision Speci1l1fmS and AS TM substitute seawater had been Ilsed ill 1I1f' prcs,mt study. Crack growth data was acquired using the Alternating Cllm~nlPotenlial Drop ledllliquc. The dala were then analysed according lo tIl<: ASTM f('WI1IItI('IJ(laliou£(iH,1981. A potellti,'l of ·830mV(SeE) wa.s applied flilrillglhc ci\tho,lic prolcdioJl lests. The loading frequency was :1111,in air and 0.16"1111, in sc<t.water. The air tesls were conducted at room h'lIlperalllrc and the seawater tests were conduded at 5°C. The stress ratio fur allllit~tests was 0.05.

Multiple tests cundllcteri in similar envirolllllclits produa:d highly itgree·

;tl.[,· Tl'sliits. Pree ('orfllsion crack growth r,'ie for the illtermetliate range of6Kwas ahout. 1.f1lo 2.0 higher than that in 'tir. However, applying a c;\t.hudic protection of·8:JOmV(SCE) reduced the crack growth rate to the kvel of cr;"ck growth rale in air. For all the corrosion fatigue tests (both ca- tllOdirally prot('ctetl.'1H1 b.'Cly corroding) tl.K u, value was fouwl to be higher lhan lhat in air.

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ACKNOWLEDGEMENTS

Tlie work forthis tllt'sislI'ilS"arri,'d nul allh,·Fal'ulLy.JfEngilll'('rilig lind Appli,'d Sl'iellcc,~1('lllmilllIillil·,·tsiLy of NClI'ftlllllll[;lIl<L FtllIclill~fur the r<:5c<trch II'OIS pfO\'i,!<'d II)"Memori;\1 lJniwrsiLy of N,·\\'follllll!;lud. , am especially indebted10Ihe D,wI oflill'Srhooillf Graduate SLlldil's, Dr. I\lill·

pllS and the Associate D"all Dr.C.Shi\rp,~fornlVi\nlill~1111' 1,11<' f,·II01vsl,ip.

Iacknowh·dge the £illallt:ial support provid('11 hy 1.11<' N:I1.ural S,'il'll("'S ;111<1 Engincering 1l,'seilrt'lL ('oIllH·il of (',\lHlda Ihrough anop,·ral.in~n";I'arc·h j.!;tilill to Dr. \..:,1\'111lIiL~Wa1l1Y,wlLidl Illiltle it possihlc to l"lury I>lll!.Il"n·""ill"'!'.

IlUll grilldul toDr. I":.~lllIllI.swalllYwhohilSactl~d ;L~my slIlH'rvism and provided active help, advice andgllidll.lll'l~lhrungholllll\l:1'(lIlP;I~of llll' rc5carch. [1I11l~talso 1.bankDr. It. M. llopkillsfor his hdpflLl illIlU1.1i illUn' field ofcorrosion 1\1\,1C(jIT(j.~i(Jll fati~IIl" Add",1thallks toDr.'I',II.PaLc'l fromthe Dcparlment ofBilldH'lliistryforIll:linp; liS ;UI;lIrvisor lollworp;ank part of1.l,is proj"ct.

~Iythanks arc also cxtcnded to thc TechnicalSf'I"vj(oes forflll,rimLill~L111' experimcntalset up Lo its exactinp;n~(lllircJllc!llsandtoLll(~tf:dlllkialls c,r theStructuriJI Lahoratory,l~spI:ciallyMr.Alistin Bursey,fortll(~ir ':"ll.~I.;lIIt help during Lhe period

or

tile stlilly,

iii

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'"

_·Hi

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.l.~.::! Crack gr(j\\'th rate evaluation

·1,:/ C'alil,raLioll 1.·1 Ik~llll~.

1A.I ,\il'l,-,:<L~

1A.::! Freecorr()~jul1 tc~t~. ,1 A .:\ Cathodk IHOLl'Clioll tesls ,

·l..'i Discllssioll of tht' r('sulLs

5 CONCLUSJON

!i.1 H"'"UlllllH'luliltion forrllrLhf~rwork.

References Oibliography Appendix

48

·18

-I,

52 52

.'3

T3 76 77

78 85 88

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List of Figures

:!.J An idl',d ''rack fl;WW1.11rak"lIn',' .

•) 'I Bl'~pira~iollprot'!'ssillslllphiltl'rc',llll'illj.\hal"l"ri,l :1.1 G"llI'ral ,'xperilllt'lllal sd lip.

:1.1 I\STM(\)lIII>;'C1 T"I1SioliSI','c:illl"lI :1.:1 LoadingilrraI1W~lll'·llt.

:1..1 MTS Hydmulicwlilrullt~·.

a..'i TIle' Crack,vlir.roCallW:;U,,[~llJTt,·sl.ill/.\ S"rLWiU".~nt~·ll.

;1.6 LOIul control flol'l drillt.

:3.7 Three voltage dHlIllids usc..1 in"el'!)lIlt';l.~lln'III'·IILsV1-",wr crCllce, V 2=,;r,!ck;lllrl V:l

=

total ",,1t;',I!;l'ti

:J.S Schematic!! or tile ,lataa':/]lIisitinusysll'1tlIISI"!.

:\.9 Seawaler lcsllil'LlI[l

:3,10Sid,:vit'w of r.orrusiullraLi~Il':Ll'slcIlitlJll",r 3.\1 Top view of corrosiotl fatiWll: 1.",,1. eIlallllwT. .

:\U

:Ui

:\7

·w

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C! Cril.l:k growth,_;;~~.~:lor: airI

·1.:1 C'riu:k growth f;llrvc for: air 2 01..1 Crad, growthcllrv,~for: air:I 'I.~ Cra..k~rowthcurve for: seawaterI -l.G CrilCk growth Cllr\'e for: seawater2 -1.7 Criu:k growth curve for: seawater3 I.S C'rllek growth ';1IfVe for: c,;,thonic protection [ ol.!/ Crark growtht:l1f\'l~for: catho{Jjc protection 2 ,UIlr\:rt(~l:sp"(inwlJI.

·1.11Air t,ost: Slll',:irm:n 2.

,1.12,\ir L'.'st: slw,;illlen:J.

·\.1:1 Air test:13,~sttit line..

.1. H S"llw:1Ler fn:e corrosion: specimenI.

.1. I.e. S";lwater free corrosion: specimen 2. . .

·l.UiS,'awat.~r rn~c(orrosion: specimen

a.

.l.l~CaLhodic protcct'on test: specimenI. . .I.l!.l C;tthodic protection test: spt'cimcll 2..

·I.~llCathodic prntc"liunt"st: B('stlit Iillt>:;.

,\.1 Lil'"1,'st :;.·tup :\.:! N"1I1ri<:IlL L.-:;t:Spt'l'illWIloui,).

:\.:1 Nlltri.mt tt'st: specimen tll'O

,')·1

.')(i

·IS

.1'

lin 61 62 li.l (i.]

6:3 Uri 07

69 70 71

89

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AA Nulricilt lest: SIH',·iln"lI OW' and two

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List of Tables

I.J ,\lal"'rilll('''Il~1 alll.~ ;jl

!JO

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List of Symbols

d,l/.IN

~nB I\CrO P,,,,,z p,,,,,.

"r

R p CT

v,. v.,. v"

v;,

V~,

v;

Salm:.ll'.1 ('alumd Ek"'lr,,,I<..

AlI1rrintl1 S,l'"i.·tyf"rT,·"tilll=:"fMl'lo·rials.

Thf'_~h(,I,1 ~ln'S.~iut"lIsity ( t ..r.

;l.lil.<illllllll"tn'S..; int'11sity ( I"r.

Millilrllllll SIn.,."lllll'n"it.,· (•..-h,r.

Crit;"i,1 stt...;iult'lIsityf;...Lu,.

Sl,..~s illl':Il.~it.v(;If'tllT.

CrackIClIglla.

(.\·d"I'\Jllul.

C~ark ~r"wLhIll'r 1I11it'"y,-I,.o'riwk ...:r"wl,1trill", Slllphall'11",IIl<"ill~l\at"l,·ria.

Alt"TlialiuJ;(~lll'n'lIl1"""llll;l\[In'I'(L,~·lllli'IIIl'J.

i\-1axil11111111",,,I.

:-..linillllllnl,,;\.1.

Luacl rim)!",. (1'" r·I'_,.. ).

Sln'S" n,liu,(1'

rl'·....

"I.

era."ktip Tmlius ..

r

rmvallln', (;UIl11H1C(Tell.~iull(SIH..ocillll'IIJ.

PuL.'uti;1! ,ITul' h,1.w'.'l1 "r"h.'S illq ;...k l"Uj!,l!l;1=11.

l'ull'lllial drOll 1...t."1~·11prollll'S ill nilf"kI"II~UI;./11.

IJislltlH"I' 1"'lw''('11rlf,·ro,,,,·,,,li.~l;Il''''·( ....r'·r.·r"·'·,li~t.UI'·'·"

Sllrrll"'~disl;.lU·O'I"~"""'('ll1tdi"" prol",:! ;,l'·r;...k1"II~l!l ~1.'11.

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Chapter 1 INTRODUCTION

1.1 Corrosion Fatigue of Offshore Structures

[)Ul'to allltlllll'l~rof WIlSOIlS n>fI"osioll fatigueplaysacrucial roleinthe failum of1h.,offshore~1.rudlll"cs.,\ typical olTshore structure experiences a lar~,'1I11IH]Wfof {ilbolil.]07)vcry lowfre<plency(average 0.167 Hz) loading.

Thi.~,ill nllljllllcl,i,,"lI'il.hH.-Iu'lllically active a'llicous environment CI\I1 pose a sl'rinns thn·al. lu allYsll'II(~tllral ~ysl(,lII.Offshoreslructurc.~are usually made of high :4n.'llgth sl.c'cls wil,1t good toughncss characteristics but are very much pr()nl~10 Ilytlr0l!pll cllllJritt.!Cfllclil.

[,'adorsalr,'t:till~Ule corrosioll fatigue resistance of a metal can be of 111l'\,al1llfj.!;inll. 111l'dL:llli('ill orellvironmentalorigin. Among the mechanical l'ari;lbl,'s whidlg()\"l~rtlth"ITilt"kgrolVlh ratc arc the loading frequency, wave rurlll,strcss raLio 'Iudtlil'sll"t'ss intensity range, AI( " Environmental fadors ilKhull' cl1virolllllcntal rhcmislQ', pll, temperature, electrochemical potential 0I1111101"i11organic activity (hio fOllling).

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Itis knowil that b1\('Il'l'ia (cSIll'cial1y~lllphal(' n~lll('ill~h'H'h'ria) haw a considel'ahk' "bilit.y 10 clihauCl.' corrosiull [,lli,::ul' ill lllilril1l' strut'lnrt':', S,'\'·

eral models lla\'c hC'C1l suggeslcd III ,'xplaill lilt' lllit'whi"ll'llrmsillll fat.ip,u,' phcnomena. Acconling lo UIC dassiral thl'ory, the1l1l~Lilhnlit·"OllSI11l11'1,j"ll or hydrog(,Jl hy I!If'b;1flf'ria(";!lISPSd(~Jl,)lilrii(iltion(aUtI"llllalln'tll'f)rro.~i')Il)ill thecr,1Ck tip. 1I,\'dro~I'l1 ~1\Hidl'Oil I hI' u1.lll'r haud ('an poisoll tIll' hy<lrol!.'·\1 rerOlllhina1.i'lllrpartiol1 l"PSllll;ll!'. ill a I';l"<'al,'r l"·fm.'at.ioll llr atoll1;" lly.lro,·

[[0\\,('\,1'1', thec1as~ic'a[thl'Ql")' fms <:01111' into~l'\·,'reattilfk mainly (rolll1'1'- sea.rchers like I\ing and i\lil1el' (I!/il), Illllt: of lJiologindlypm.llI<'.~1slllli,I.' ill cnhancedcorro~i(J1Irate has Ilt't:olllc importllllt. [>ossil,ilil,y

or

phnsphat., ill tansillg llwjor ('orrosioll lias ;llso1)l'(~11invt'l;ligalt'(I,

Given tlwhackgrolllld

or

tIlt'"OITOSillll f,\tigu,.' l'whl(:lll in "Ifsltuf".~trttr·

lures, a reallIt,'('dclllcrgl'.~lo olll.aill moreIlualltitativ,~klluwl".Ip;t·ofUti' problem. Thiswillgiw: rise to a more dlkil'lIL IIWilllS to,lt~i~1l t1)l~olfsllllf(' structuresas well as 1.0 pmkd 1I1l'tll.

1.2 Scope and the objective

[tis ill1[lOrtillll Lo Ob\.ilillI.ltf~raliglu: prupl'rLics of olrsllort: slrndllral stf·.!ls ill itscawater cflvil'Ollll1Cnt,TIIf'(I'snlts shouldheilliIrfJ(m diH:dly,tppJi";IIII,' in the disigllprocess. Abo, thetl~~ts ~ltunlllCOliform to ,\ widdy,":f;f:l'l,~,1 slalJr!<m!. Tllis allows coll1parison of LIn: test rcsults and lliliLlitatiw: jlltlW!'

10

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1I1t~lIt'HI Llwl".rfonlliUIl'l~"fllll'meLal ill l[llcstion. In the current study the rilti~IH'[,l'lIavilmr(,f0111ulfsllort· sLrul'l1l1'a[ sleel,CSA G ·10·21grade:l50WT, isiJl\'l~sl.igal,'(1IJlIlI('r n,rrosiw,~n\'il'OllJlIclitS.Particular attention is paid to llll~df,·tlsufSl:awatl~r[n'l: cormsiofl aud cathodic protection on the fatigue l'rlll-kp,rowth ratl'.

Tllf~fatip,lll"1 .IS aT!' c;al'ril'{l oul ill till' following cnvironments,

I.:\ir,

:1.AS'I',\I s";oll'aL('I' witll ·S:lllIlIV(SCE) catl'udic protection ill5°C.

TIll'to'~1sO'lIlI' wa" d"siglll'dd,~~('ly[ollowing the,\STr-,1staudardE.G·li ([~ll'il).AS,~I{"IIIII',ll't T'·II.,i"n.~p"CiIIICIlwas choseoasthe t('st specimen . .'\ST~l ~YlllllO'li,' sl',II\',ll"r (AS'!'M,U~'Signatioll:Dl H I·i5,1900)waschosen ,IStIl<' 11I;lin Ill('(lillllllluringIII"l'OlTosiOli fatigue tests. At least two tests

\\'l'I"l' l'ollllllrlt'{l ill t'arll l'lLl"irOIlI\$,lllil! <:IlIlditiollS to check the repeatability ofLIlt'fI'SUl1.' amIrll1Ilinllthe cffcrti\'clles5 ofthetest setup.[tis expected tll'lt 1.111'<liltapublishl'!l in tl'rllISufIlle ploL:> of the stress intensity factor l'illlge(~I$ilildlhe<:Tackgrowth rate(da/dN)wouldbe agood source of fal,i~lIl'dl·sip;1I Ilill,l fOfIIH' ('SA (: ,lU-:n.grade3[IOWTsteeL

TIlt'!in'sPllt Sllld,l" is a pl'I,,'lIrsor10alllllhcr sel of experiments invcstigat- ill~Ill<' dr,·,.,of slIlp!l;llt'n~llll..illg harlcriilonfatigueofsteels. They include (,'slsill I) l1ul ri('llt ridl SI''1wat-l'r mill 2)~cawatcrinoculated with bacteria.

II

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A.

I:::!

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Chapter 2 LITERATURE STUDY

2.1 Introduction

(~f!rrosioll fillip;II(~1l{'l"Ill'S ill Ilwlills as aresultofthe combinedaction of a ('ydi,'sl.n·ss alld ;(t"tlrnlSiv,~':II\·j.'Olllllcnl, At the simplest level it r.an he al.tl'ihukd luIIH~anvilir ,lissnllllion of the metal mnss at thecracktip. But ill tilt'ITil1lirl'itflillhe milch l1Iorc l'olllplicnLcd than that. Accompanying l.lwdi.~solll1.illllwart1011 illtill:rr;lrk Iip arctwopossible cathodic reactions:

I.t.ll<' n·dllrl iOll of,lissok",lC).~.l"g"l110hydroxylionsand ., till'rt,dlll"litln nfhplroW'1I ions to alomic hydrogen.

!':rf,·cl.\Jf tIll'first.n'ndluu is prolt'ctil'c in the form of metalhydroxide deposits onI.ht· 1l11'lollsllrfill"p.TIll's,Ot"mllln~ilction.onthe otherhand, is the most ,[;uHagili/,; 10111<' 1l1l'1n1. TIlt' alOlilic hydrogen adsorbed into the metal surface 111;1.\· slIbS''tpWlllly ,lilrlls<, illl.ll lIlt' md;\II.l11i<:e atlhc zulle of hightri-axial TIlt'high l'quilihriul1l pressure

or

hydrogen inside the latlice affecL.i

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tlw illtq;:rity of till' 1Ill't,d. making it SI1Sl""I,Lihh, III "1"",.ki1\,\.\ illI"\\TI'1",,,1 l'IJII,litioIlS.

InSIlIlI111,lry,tlw h"'illiz,'d dissolll1-iull (all"di,' 1'l"\I"lioll) at. III<' ''I'O\''k IiI' lIlayn>:;lIlt in a higlwr fat.igll" <'I'il..k p,l'uWl!l 1',11<' or 1','dll<'l.i,," ill IIII' Silll!"

(liarty andN,jd. 1!/!lQ) by 1t1,'i\lIS of <'I';".k lipI,llllllin~.On1.111',,111<'1' It;<II'1.

tIll' Mlsorptioll ofhydrop;clI ill111t~l.1Cill nd,IIl"I"slila,\','ans,' lIll'l;d "11I1'lit.

l!t'UWllL alwalloftht~<'rill-k tip alldlll'l1n'ahi,~Ill'rni",k1:1'0\\,1.)1 1';11<',T]ws,-

all IIl1dt'rslillltlinguftill' lIWclllUlislllS illv,,!Vt:,1wld,.]1 al?,ailliln'fll1ll'li"lls"I'

liollUllLIIl' faligHe bc1mviollr"fstl!l'l. Wlli]" ,lpsi/l:llil1l?, il fatil?,tll' "XI"'lilll"IIL, Ollt'has tilI'h(JOSl~'(fl'w pilTall1dt:rs for til.: l'x[wl'itll<'t1l,lik..,1,"llIl"'1'aLlIl'I\

lo;ulillg [requency,tlll~J,:vclof ""t!tudi .. I'rot,'diollde TII"YIJitV'-L"Ii" inan aCCl~lltahlt~lilllit. Also,knuwbll,(l:"fIwl\'tllI'Sl'l,amlll,'I.I'I'S Illayafft~'l,till' fit tiguc hehaviollr, is hllporta/lL.Tlli,~allows pn),lidoli

"r

t.h,· fittil?,t1" l>d",vi"ll!

of tile Inetl,1 inItdilrcr('lIl1:llvirorlllwllt,AI"l(IfworkIta~1""-,,,1,,111' illtill' pasl lWOd,~ratlC'Slu lllldl:rslam]literoll: of llll:s" I'ilrallll'tl:rs,ShIll"or wlJidl

1,1

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2.2 Fatigue crack propagation

Wlwn till' CfMk growt.hr;~t,~(c1a/,IN)is plow:d against theslr~>ssintensity f;lI'tor rall).!,"':"· in II11)1l;.lo~ sr;lh~Lhe ,:llfW shows a sip;ll1uidal wlaLiollship (Fill;. :!.I). Inrl'~i()11 1,61\ asymptotiC'<llly appl'uadll's a tlll'fshuldvlllll~' 61\,/.. 1",luw Wllidl t.11l'r1' is virtlwlly II" ('l';u'k gruwth. llf'giOll:! shuws ;\

litll'ar st,'a.ly st"tl' ':l'al"k p;rllwlh Lhat call lit' ,'Xl'T('SscllI)ymany iWililabll' r..liltil>lIships (l'i,ris iwd Enlug;lII, If.l!i:1) , Itegioll:1 llepids rapid prog..l'SS uf

'1'1 ... lIi1tlll"l' urthl~l'riu'k growth ralc curve changes ill ;1 corrosive clIvi- rOllllll'llt. Fur luw t':t1lI ...r,,01,~t.",'(sthere is ,Ill IlflWlInl shift of the luwer part uf t.l,,' l','gioll:!.This 1Il,'iItIS i\low,~rul\u,and lower slolw IInder corrosion, This Lrl'lld l'ullli'IlIt'S wil,1i (owl'ring ufth~fn..'(luellcy.

llil!:hstTl'n~thsl,'{·ls sl·lIs;1.in· to stress l'llI"Tosion n,Kking art" alfl'ctl'd hy a "l>t'I'lIsivl' l'l\viTlllltlll't11. nnly allo\"<' a I'erlilin slress intellsity range, 6K/$cc.

'I'll<' \lillllt' ..r j.I\,.'wr..' gill'S IluWll with dl'<.'I"casing rn'quelle.y, FUf,~OllU'Illt'lal'I'livirUlll1l<'llt Spt"111(~'g,titililiulli alluys ill chloride ions alld SlTlll'Lural st...·1 ill hy.lfugen gus), regiun 2telltl,~tobt~horizonlil1.Snch11 purli"n ur llw nIH'" is ""lkd llw plall'illI rt'gion, Tlw wi<lLh or~he pla~l'n\lin-

<'rI'ilst'S wilhdl"'rt'~lsillgrrt·(j1Il·l1fy.SlIdla plateau formation usually illdiclltcs

15

(25)

hydrogcn C'mbritIJI'ml'J1L

Thc following subse<:tions hridly intordllCl's dilft'l"l'nl par:lllh'krs;Jtft'dill~

acorrosion fatigue ('XPCri1llClit.

2.2.1 Cyclic frequency

Normally,1l.lower frequellt')'C'l\IHlIICl~sfatiglwnack grolVth r;ltl'. l'al\il.~)'lIk and Romani\' (1980) ohscl'\,(,(J thatlow and mcdium Slrt'ugth slwls art' ilf- rected byst~awalt'ronly lit 1011'1'1"fl'(~qIIl'IlCYrllngt'.TJlI~crac.:k~rn\\'tllrat" at.

IOllz was almost tllcS,ll1ll'il.~in air. TheraLl'illt're;JSI':'! su:,l<lily as fl"l'IIIll'ru:y drops10 I Hz. belo1\' whichthcl'f~is 110 fllrthl'r erred uf fn'lluell':Y.

Ollchi 1 et, 11.1. (I!)!JO)Sl1~t'sle..llhat the highri~l:timea~.'<IJriat"llwith a lowfrcqut'llcy IOlldiuK ('IUISI'Still'h;~her<:rllckgrowth. Tlw IIwdmlliltlll rcspomihlcis hydrogen ('mhri1.llell1C'l1l, whidld01l1irHLtt~Sover ;\llodk IJissn- lutioni~ttowfrequency.

2.2.2 Cyclic wave form

Ouchi 1 C'l.:II. (I!J!IO)com[laf(~dtill'r~lft:d(Iftliffewl\t IlladingW,WI~forms, It was obJ>er\'ed thatWiWl~formJ> withhi~hrisetimewillcaliseI1Imt: (1'lll1ilge.

The holdtimeand lhefaJ]time halllittJcdrcCl Oilthecrack growth rat.:.'l'llili observation seems Lo be supprJl"tctl hythel1udings ofPana.~Yllkallllll"malliv (1980). A sinusoidalw1\vef01'l11produced similil.r wsulli\.~alIyJIllTldrictri- angular wave form.Positive smvlI,oth W1\ve (rapid loading) andsllullrf~w;w..:

made the lIH1lerialinsf!llsiliVl! to COl'rosiorl fatigue, Hilt, ;,11(~gIlLiv<!sllwh,utlt

16

(26)

d~/dN Linurcr~ckIrowthr~t

Rcsion 2

F'igul1'~.I::\n ideal crack growlh rate curve.

(27)

wan' (rapi,l 1l111,>acliIIKlll1"Id,' Illl'~1'''''I,~''1\~itin'10 c'orrtlsi'lll.

2.2.3 Temperature

Ouchi2 ct, aI" (19!JO),llol,icl."dlhat dr('d of W,l1.<'rl,'mp<"ralurc'(:Ulg alul 5°C) on free cOI'l"osion fatigue stn'lIgUI was <,'xtn'mt'ly slllill!.VC~llUlolUpalllll<l Putatunda(1990)obsc'rvcd thilt tilt..'crackillClIhati011 jll'riod decrt·..l.~'·swith

2.2.4 Cathodic protection

Theeffect of cathodic protectionOill'Ol'rtJsiUllfiltigll'~varies wiclc·ly, C"rrosioll pillingandhence crack initiation('QuIdIw delayed hy t"illhodic: Ilrol,'cotiulI, But it has hl:l'll ,'ollfirn(l!c! that a llIon' nt'/;alivl'mt!co<li,' plll.,·nti,,! jnn"as,·.•

1111.' crack growth rilll'.

~'liUlhll(J!)::!I) sho\\TIllIlidllll~pUkllti,,1 d llic IIMn/w rrm'kIt~\'t'lis lIilrcrelil from the bulkcll\'it'ollll1l~nt,The reduction illpott~lIti'll insiclt~I.Ill' crack tip was mainly aUribult:<1 tu all ohmic drop c;lus(.'11 hy ithilldC~f(·tlc!LarA"

(low in and out of the crack tip.COIISC{[lIl~ntly,this lirop shouldd(~pt:lItlun the crack opelling an,1 till' widlh of Iii" SPC:eillH:ll. IlruW11(1~1H:1) IJrl~sc~lIt~

a.summary of studies 011 11)(' "If,'d of lIarious catllodjeJlrotc~dioll h~lId~Oil crack initiation and propagation of metals.

Jones and l3lackie(19!JO)ohsc'rlled that, with 11 fewc~xcepliolls,c;ath",lic;

protection reslores the corrosi'lflf,tli~lIe lirl~10 the :,irIC~lIc~1.Tht)drt~d"f decreasinglhc~ poten~iidfrom·(i()OIllVlo10!)OmVvaried from sled typ.:~o

IS

(28)

:>tl.:dtypl~;trl<JfroJl1 plaill spt:cillJl:JI to lIotdwd specimen. An optimumperfor.

m,UI(l~at ·S,'iOIIlVWitS nhsen'cd for most of theplain spccilnl,,'n and notched slwdlfWll. Tll01lgll, 011 lowcringlhel)Otcntialto -lO·')OmVcaused. furtherim·

l'r(lvc:lm:lIl illf"til;lIl: life uf a ]>ilrticular notchcd!3S!J,Ograde817M40steel.

1.'llji,Iud Srllilb (1!J::):I) obsl'J'I'(!d tliat the a<!lJl.'IlUS environments gencr-

"lIyf:auSC'1i;~ hi~llI:r flltiKll(~ cr~ckgrowth inf1Y·laOsteel. Thefatigue lH'rffll"lllilllt"l!bC-"'illlH"dO~I'1,0that inilir at \'f'ryhigh .6.1';valucs (of about

~lIl~lPa";III).Hajpatlwk andIhlrLl (I!Jl:\8) tested nine high strength sleels for their mrro:;iol1 f;ltiWlc cr:tek initiation.Acathodic protection of·1.10 V (S( :E) improwdthl: fatigneIH-rformallcc. At higher frequency the specimen il,.hi.:vc:.l the pl\.lllrnlln: limit. The free l;orrOllion cnduran-::c limit was fOllnd to he les.stJliUlthaL in ;,ir for all the stl"c1s.

'I'uhhyand Booth (1!!!l2) ll':itcll single I:dged notchedspedmens made of high :;tf(!llgth :;1.,-('1:; S/\H 60 and MACS. SAR steel specimens testedin sea Wil1.1'1'without pl'Olf'ctiuli.~hll\\'ed;1.crack growt.h rate 1.2 to 2.2 times the rill" ill ilir.:\f';,lllodk protectioll of -S.'iOmV at 5"C reduced growth rate I,,-lul\' air. ,\1.~lloClilt."nark gl'lll\'th ratl'Sarc found to be very close to the

llh('(~iHld 1)1'lgildo (1!I!l:.!)tc~ll·llhigh stfr:ngth API 5AC C-90 riser cou·

pIing lll'lkriillI·onlpitdLCIl:;ionspet:imcnsinair, seawater and brine envi·

rtllllllCIlLs. Thel.Tilt:k gruwth ratl.' in cOl'fosive cnvironmenl was signilkantly hi~llt"rthilll thitt in ail'. TIle I'fr<'cl was more pronounced in the mid .6.K

HI

(29)

region thall for ('ilher .. \'l'ry I'l\\' II!' higbj,1\ \';([11'" j,1\'h \';dlU'illrn':I~,',1 with l.he ,Juration \)f du'\"SIs.Il1H~1probahly IW"illISC' ofhi~JU'r,kflusil i"u of cidcnrcous Il\ilkl'iab. his illllhlrlalll l.u auk that, a ralhodi,'l'rut,~'li"ll of-;:;.50mV(SCE)c1l1rillg thl' corrosiull fatigll(' lc'sts f:lilt'd 1.0kill~dllll'll tIll' n:,,'k growth mte to the~ir11'\',,1.

~lorgllll"t al. (I!J81',"olldllcl,',l mrrosinll rali>;Il",'xll,'rlm"llts Ull sCrO\,'·

lllraJ stf'd. 'I'll,' slr,'ss r,llill haollhl"tr'~'lOilf;lli~llt~rra ..k~ruwlhf;llc'S ill air 11111.higlwrHralio ]ll"l),IIl"1'1hi).(I\O'rniwl;)-\wl\"thral(',~inS";I\\"II,"r.('al,ho,li,'

of -0,TV (Ag/ Agel)wa~^(Clllll,1ttlIll'lll(~ ()~ltilnlllli. illCJl'c~llC'P;ill.i\','pfol·ti'lll potentials I"C'S11lted ill ;111~cCf'I['ratl'dcTilrk grow!.ll,

2.3 Sulfate reducing bacteria

hYIll'ogl'nclllhritllclllCrltof llll'lai. Tldsst~cl.ioll dC~alswilli thdrIIlltritillll,d pattern nnd their possihlcrokinmicrobial mrrosion andliydroW~lldalllaPiO:, Slurs1\N:ha.~iCidJyItOIl-pllotolmphi(: Hud ILc'luin: roo,! frurll llfWlllil' Illl·

tric:nts. They arealsoan'll:rolJic :Itltl !lSC' sIlJpll'lL(:.~~llw c:b:1.roll iUTl'ptor.

:w

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2.3.1 Nutrition

It is illll"'rlilut III 11111" (lIitl:lilltm. 1!k).1) tlllll SIlW:s ha\'e a \"ery limited 1l"lrili,,"al sl""'l rllm""Ilid!i.~tlll'rmooyllamic in origin.The spanillredox l'ul"llli"ls ur1I." d''\:lwlI :u'rcplor '1I1l1 dl.'Ctron donor gh'Cll a measure or the

"lll'r)U' it\'lLilat.te tu LllC org'll1ism rrom lher~pirAlionproccss. Considering Ily,lruR'!11 as till' d,'I'Cmll ,lnllur l<l1d sulphate lIS thc acccptor

snn

prOdUCL'lI ,'Iu'rlt;y still rarI,·sstlll'":11I;II'wllP.('onscclllr.nlly SRD'scan only ha\'c an :H"·l~II\.'lrwil I, n"I'I;( IUll"lltilllI"ssthan·I I GtllY10 prolluce any energy.

It.lias 1""'11 "lIl1!iftlwd t.lwlll1011',·ltl1\1' hydrogen acts as the ck'Ctron donor.

SO~-+,1I1~... S'l-

+

·1ll,O

('.11

'I'll<' n':u'li"ll is .'itrric..1 uut byIUt"l1l:}'UICt:alk·d hydrogcnn.se.

Fi1;nn' :!.'.! Sdll'lI1<1lirillly shows tIll' rc.."Ipiratioll pMC<'S5 invoked in lhe slIllll.alpr... luc·in...I,:l.'·II·riil.

2.3.2 Microbial corrosion

Tltl'wan's'·lwi.1/lropo.~I..1 mod,'ls IryingtocjC5c:ribcthcmechanismor COr- rtJSitlll ,1111'10Sl1tphilll'f('(lllring 1I..("I(·rill. ,\ccording to the classical theory wl,i..1tW:ISpwpos",jh.l· Wol:f.CJloil·!1 1\llhr all<1Vi\nclerVluglin 1!l34, the SRB an'T1~llOIISilllt'rOI' ,1{'<IthOtlkdt'ptllllri~o1tiOlICi\\lSC'C1by an uplake of hydrogen fur 1lll'ir 1I1<,1 .. IHllislll, Tilt' following~"ril'Sor rCl\clions oltlline the c1l\SSicl\l

:!1

(31)

ATP C~tbo"sourn

EI~ttro!1I G.n."I.d

~'\111 .

"'''

C~ubor.lm ElectronIr~nlpo<t

Figure 2.1:RI'SjJirnLiolipmc~sinsulphlll!! rNlm:ill,lt h.ll·lI-ria

lhcory.

"no(lieTcl\<:li,,":

CalhOllic n';\l:t ion:

Calhodic dCl'olarizalioll

hy snn

iL~ill (<-'(pl.:!.!)

(2.2)

(2.:1)

(32)

Corrosion prmllll:t:

(2.5)

(2.6)

11-',,1+

+

,lll'IO -.:!Fc(Ollh

+

FeS+20W (2,i) 11,1\\"'1','1'lilt'dassi"altll(~llTis Ilot withoutit'sdcfieiclleicR,Theratioof

"(Jrrn,I,~1iron1.U ironslllphi,lc accordingto this theory(~:l)does not match wilhthat fOlilid ill actualCil~"SPO:I).

1\IlIiljur chall(-'lIgl'faillefroUlI';ingandl\'li1lcr (1971) inthewake

or

the ()11~('rI'at;olltil'll:

• tiler,·isIU)IlirN.'ll'l,);,tiolishiplJl'lWCCn hydrogenase activity,theeau~e u(d"llol;lriznlioll ilcl'onlillg to thccl/lSsiealthl'Or)'andthe rateorcor·

roSiOll,

• inorganinlily pmdUI'l.'(l iTOlt sulfide itselr was found to depolarize ca·

thodir sled in ilh:U-lcrill fn't.'~ystl~rn,

Thisshif1t,dtill';lll"lIliulllowards~l111idcastlicmajor causeofcorrosion, tlwughtheha.,il'sl'torrl"l("liOIiSpropoS(.oQby lhe classical theory remained lIlu'hangp,I,

23

(33)

r\simulated \'orrOSioll rX[l('rillll'l\l hyh'l~r,'Hln(!!)G(;) fail ...lto r.,.'.mllll<' scverit}, of a nel,lcorro~iolltl'St. III fact, a very lillIe trace of rl'(lll,',,(! sulphait' was noticed, lwrsoll all,l Olson(1!)::tJ)trkd to explain IIU' "\'I'lIl init{'UI1l' plctcly IICW light. Sen'ral phosplHll'llUS CtJ1I1POllllds (like.H'11')W,'rt:rl/lludas the universal corrosion products ill\'\)king bacteria, ILwa.~ SIJ1~g('lill'dthill, a highly rrilctiv(' and volal.ilt'1,lw'~llhOl'II,~('ollipollnd IYhkh prodn('l'li iron phosphide lInrlllhosphitu: among utlwr things. is iletllally rt'spol1sihl,' for Ill<' COITosion,

Sidvart:ZZiI aml \'idt:la(I!J~U)I'ondlldl'(l anotlic1l1)[;lriZ;~l.i"ll11'St.1' 111111.,1' the elfer.t ofSHII,The I/p·,wra[I'<1 salt SUl1l1i11lll'urYI'!rollllllrt~~ (It,~ljll('trI,'.

giOllS, Tlwregiolllu~tw.~:n.(j(j[J11iV lt1-GUOIllVslllJlY.:d II tll'Crt'asl' ill111<' cllrrenL bCCilllSl' of lil111 !,i]ssiv,ll.iOll, 11 11 111'1' ,11'mtl,1 r.olltliLi01I 110 sud IPiL'l.~iVlI' tion IYns noticed, lIowt:vC1" in hoLhtill'('ilSI'I'pilLilig polt'ntial Wa); itlt'nlkal, ahollL-580IllV.\VhclL lhe same'~Jl(~t'il1ll'n WiL~It:stt:ll inSf:'IWi~l.t'rf"f1ll1..t1l1i- lIate<1 wit.h bllClt'ri,l,<til"hS"llCl' of ;OIYp'ls.~ivaLiollalld ill]tlvl~r.dl rl~lllll"li()1I of pitting !'otelltiiJ1l.o abollt·TOll 1l1V WilS o!Js.:rv('fl. TIlet:flllfl~lltmtilJllof sulndc ill Llle bulk solution Iwl IIII(Jlit:('alJl.~ dr.~ctunLIIl~illiOllit; adivit,y.

Higher sulnlle conccntrationilll;l'I~;L<;t'd<llIodir.r.lIrrt~lltalii Ilow(~r'~tl pillill~

rotcntial. The dfcct WIlS f'venWOI'SI~ulItI('ril111~·at:rnLI~flconditiO/I, II. WitS concluded thatSRncllhllllrcs "urrosion lJy n:rnovillg pitssivllLioll film iHHI producing metabolic,~lllfidl~,

[)alllllils 1'1. nl. (1!1,~RIsllldit,,1nJrrosillllilltlHt:f~1Ihys'lllJhill'~ r,~r1lU:iIlK

:H

(34)

harll:ria, " I,alll~,)'ili("Cmlitrurll~lout oj two half cells containing the same IlilJlu~icilt1lI''Ilia(tu lillpfll,rlSRnlift·), olle heing sterile and the other being in"c.lllatl'll willth;l(;tl~ria,Tlli" ,,1101\'('11 ror testing the effect or both hydro- W'IlMi"ill"livily (llxi,lalillll 'Ifh)"llm~I'IIJand 1I1S.I'nother batter)'wasmade fmlll I, sillKlo:<:t'Il,Ou,:of Llw '·],'I·tro,I,.,; IVilli covcn.:d byi\Ilyali:;is mcmhfi\ne whidlallml,,'dII~S,tilruliioll hilt prolcctt'll the r.IcctroJc from i\OY hydrogc- l1:llil:Iwtivil)', (:I'IIIIllL':lllilllOIltdo:orroliioll curreut was measured for i\llthe ,'as"li. In tIl(' rirliL Im1.l,'ry.tl1<'/.:('!Icilrrying the inol:\llat(.'11 mc,lium achieved1\

Ili~II,'r(11'<11"(' 111,1,1,,) llut"111 inl LIllll1 tIle lilcrilc cell. This indicateda cathodic ,lqI'IlIOiriZil1.ilJn 1'<IUlil'll "it Ill'r by sulfideor oxidizing IlgClIt hydrogenase. Ilow-

,'\W. II""fy similar Ilitr.'ro'IWI' ill potelltialWiISobtained in the second cell

wi...•1.<J11t the c!,'(·trOiIN W,'fL' ""posed to di!lSOlvcd hydrogen sulfide and lI;ul ''Illlal dq)lJliition or irall slIlIilleonthem.

Fllrtlll'f litll.ly wilh 1111' slimt cirwit ClIffCllt bctwct'n the two cells con- tirnll~ltllt,,1"llIillal iUlIllfIly,lrup,I'1I,,~i,llltionallthe dominant callsc of cnrro- simI. TIll'I"fl,\'1 uf slllficl,' was r1illlill.llcd since non sulfide producing bad.eria

~;l\'l'rist'lo IluoS.11111: 1"\'l'llir corrosion CUfrent.

2.3,3 Effect of H1S on hydrogen permeation rate

IbS, n hi-lIrndll.,t of mit'rohinl IIlda],olhlm, is often held responsible for an

"lIh:lIll'I,1! liydroW'1l l·lllhril.llelll"l1l ill metal. This subsection deals with the

TIll' '·III1'lIll,,-.1 lil ...~ssrOITUSiOIl cri\eking of sleel in presence of hydrogen

(35)

sulfide. termed as sulfiderr;lrkiu~.is ,lUrihllll',1 tohplrnW'1l'·llIhrit.III'llll'lll.

:\lllongtill' possihk'1I11'("hflll;SIllS. till'POiSOllill~plf,'('"I, ofIl~Sis till'IllCl.~1 popular OIl<".r\ ..rordill~to this tl1<'ory poisons lik,'hy,lrn~"nsulti.l,· illnl';lSl'S the hydrogeno\!l:r\'olta~~of iroll, II.lias !J''t'nSll~t's"'11that till' pois"lls g~tf1dsorhl'tl toIheS1Irr,I\'" oftll('lIll'talal1,1I'Ilallp;<:tl\l'l,illdillg\'Ill'r~y"r

<ldsorhl'dh~'drog\'ll101I1t'tai 1hilS illl'fl'ilsillg'lll(~ hydrtl~"l1u\J,·rvollil.l\".

l\cnll'diliK lu 'l1l"lli,·1' 1h'~HYhy.lrull;.'11slIlIidl~"d,sas a d,·p"lari1.l·r rat.lU'r

adsorbceJ to til\'111<'1,,1 surface allli "lIhillln:s hydru";"ll a,ls'lrl'liull I,)' th,' following l't';,cLioll.

COIlCl'tltration011hYllrogcll difrusiull illllitrapping ill slililll(,>;s sll....·1ill 'lI'idifi.·,1 salt sodium chlol'idc solution. 'I'll(' l'l'SII!tSIVI:rt~ Hlialysl~,1ill l"rllls of ("n, LIlt' concentrationof Ilydrogl'lI ,IIOlIiS Iwllll\' L1w slIrf;lre of tJ/I: IIlI'!,al "XIJOsl'Il t.o

\Vlwrc. ,1"., is the sleillly st;Il,\! flllx,IJis the dilfllsioll ClJf'lIicilml an<l "

is the lhicklH'SS of the medillll1.'I'll'! ,lifTlision t:lldfici,mlw,~~ 1I.~SIIH",,1t"I,,~

lhe same <1S t!l,ll ror pllre,1I11111·alt·d il'llll,Th,~erfed ur lrllppillp;sil'~11'm L111' dirru~iollt"lldlki':lIt WitSIII~gll't;l,~,J.

:l/j

(36)

'l'h,!(:11v;,low ill':n'ast',1 wil h hi.u;llI'rII~SCOllcentration ('Specially at higher 1.':llllll'ralllr,'S,

A~~l.rpllllr·lkdllly;unlFer~lIs(J1l(!!J8S) studied the possihle dfcct of hy- ,lro)"'11Sl1llJlriclf'011(lllill;lll' crack growth, llydrogen sulfide always gave rise l<l;111iucl"<'a~('iu till' n;lCk /!;rIlwth rate nud a plateau formation (indicative ,)( hy,Il'(Jg<'1I"lldlritLl{~lluml), Till' colTlbination of cathodic protectilJlJ(·1,0 V),lIl1lllydwl;l'lI~\lllid,'11;111llll' lnllximlllll deleterious clrcct,

1\ilw,,~hitl1al'l. al. (I!JiG)ohst~rv,--da higher susccpLibilily to hydro- W'1l"llllJriU.I"llll'ut ill hydrogen sulpllidc~olulion,The effect was more pro- tlOIJlll"'cIr'lrjulI'f'rstrain rill". low/'rpHand higher cathodic polarization, 2.3.4 Dactel'ia and hydrogen embrittlement

.'i1l1I,hal," 1l,'dllcill)l; lIa"lcl'ia ;wd llIarine roulillg in general can increase the perliwatiull uf h.\'llro/!;/'ll ill lllPtalsbylocally altering the chemical environ- Illl'nlan,l lJy prmillciugagcnl~like II;:Sknown to enhance hy(lrogen 1Iptakc of 1Il<·l;I!s.

Coll.i~,11111 1\1001"1' (l!lIH) sltldil'1! the hydrogen permeation current in I'I'l'S"IW"Ill'manu fOlllilll(. No lllajor dl11ngc in llie rate\\'/l.~ohserved for each si/!;llifkallt lLlf'lalo.llic prodllt"l inlrotlun.. 1 lo the system. It was concluded lhat Iliano fouling dOt's 1101. hring about allY damage hy their own metaholism hul. hl'lps ITl'ale all anal'rohicl~lwirOlllllclltbetwccn the fouling mat and the llwlal sllrfaff'. Tllis ill IlIrn /'IH"lIl1ragcs the growth of sulphate reducing harll'ria. II wasl"f'rOIllI1II'IIc1I~lthat sillpliatc rt'ducing bacteria among the

"2i

(37)

foulingorganism~should Ill'tn'a(c~l a~thl' llIain c;lm;a!i\'(' <11-\('111 tilillrrc';t~c' llydrogcllp~rn\t.'atiollin Ill!'lal.

Cowlillg1111<1 Iiallc'lck(IHS:l)Slll,lil'lltlll' l'lf!'d uf snlphaL"n'dll<"ill~l,a ...

tcria Oil f"ligllCOll stt....1(liS ·1:\(iO Grade 511D).Cr1ul growth ratl' il1ll1'c'sl'lll"l' of bacteria was higlU'r than ill S('aWall'f added withlI11tfit'l)t.~.Fllrt.ht'r wllrk wasr('i.·wlllllcndcdto iuw'stiWl.t..UK'faUSt'of lIle c'llh:lllc('cl ,blllal-\I'. (\Iso, inorganically pmdlll'l'd I!zS railc'"ttlsillllliatc' t.llt' sl'\"l'rit.y or or)!;Hllic'"llypru·

ducedIl~S,,,xCl'pl ;,1 \'l'rylow .:ll": '·abll's.

(38)

Chapter 3 FATIGUE EXPERIMENTS

3.1 Introduction

'I'll<';IimfJfLll!' pn::K'1l1f!'Sl'MChWitS 10 obtain fatiguecrack grm' th char- ilcl<:rislil'sIlfIlll ojrshol"(' slnwtural steel in~hcmarine environment. The ksls W,'I"!' to 1)(' n)Jlc!JlI"l".1Oilst"ll'!;;"!AST~Icompact tension specimens ill~,ir,AS'!',\!SI'i,ll'ale'!".wilh ,l1Id withollt cathodic IHot<.,<:lion.Itwas ex- p.·nt·c1th'1Ld,\la[rolllSl1fllItv,uicty of environmentscould leadto a.better Illltl"rslallllillg oftill'n'[ilti\'cseverity of seawaLer.

This dl<lptCftlc'alswith 1I11' sperimen,loadingsystem, data acquisition s~..~I'·1l1,11111,'II\'irOllITU'IlI,,1s'-lullllSt'd inthe current investigation. Table 3.1 sUlIIl11arj'l,'·s1.110'ililpur1.alll pa!"<II1l<'l.'rs forall thetests.Figure 3.1 showsan

\II',,!"all\"j,.\\'of tht'e.''1H'ritlWlllal.~t'tup.

2!1

(39)

Name ~1axload Load ralio Frequency Environment Protection

(kN) (H,) (mV)(5CE)

Al 26.950 0.05 3 Air

A2 19.6 0.05 3 Air

A3 19.6 0.05 3 Air

51 26.95 0.05 0.167 Seawater

52 24.5 0.05 0.167 Seawater

53 24.5 0.05 0.167 Seawater

CI 24.5 0.05 0.167 Seawater -830

C2 24.5 0.05 0.167 Seawater -830

NI 24.5 0.05 0.167 Nutrient -830

N2 24.5 0.05 0.167 Nutrient -830

Table 3.1; Experimental parameters

"

30

(40)

3.2 Specimen

Tim t'''tIIpac"1.lC'lIsioli(e'l').~I)l!l'iI1WIIis sclt'<:tcd for thefatiguecrack growlh ral\! stndi':1iilSi\STM [·;(i·17-M~Ilggcststhe useof CT specimens whose planar ,lirlll!lIl1iotls clLnh,: chosen proportiollallo~hc ~hickncS5ofthe lest spc<:imcn.

i\.'lol'l! over largl:r height to width ratio provides an increased resistance to out of pl.\lIe n;\ckilll!;lindbreak offduringhigh loadtesting.

TIll'SP('(;illll'lIS \V"I'l'd"sigrll:d nll{1 fabricated snch that fatigue crackSilT' L,,": is 1",rp'·l1Ilinrl.ll" to 1111' rollingdir,~dion.The specimens were machined fWIll1~11ll1l1thickst(~~lplate which was manufactured to CSA G 40.21 M :If)lI WT s]wriliratinll. The dimensionsofthe specimen arc shown in figure :1.1. 'J'lw gripping fixturesllsf~lwith these9pecimen~aredt.osigned and fabri- cate.till,lC't'Ol'd;uIC'-withtherC(luiremcnts out lincdinASTMspecification 1,:n·17-,'1.'"),

3.3 Instrumentation

3.3.1 Loading arrangement

Figure:1.:1 ShOll'S the gCllcrllllo;Htingal'1'angcment.MTShydraulicstesting

"l[uipl111'lll, (ligul"(- :1,,1)Il'nsl1sed loapplythe fatigueload tothe specimen, :\ \'('rli,'all\Ja,lil1~arr,llLg<"l1lt'lIl waschoscn, Theactuatorwassupported hy,Illuv.-rhea'!l-lll'fllH. At.lh.,.mel oftheactualor a load cellandthe lopI'),ldilll;pin wasaltadlcd, The hottompin wassupportedbyastrong

(41)

(1Jll1ld looo;iCo'l Sjlecinen Alldil'(MiCl1~inm

n U

.J B

19oo

L

lhrn!ly:

Bitnllitoclt\ocr..ryo Figurea.2: ASTMCompact Tension Sjlccilllen pinHoI'm whichwas designed to ,·Iilllillale allY kinel oftrarl:;v,'rSl~lIlotion. Tli<'

A PC based fatiglle\(~tillgsystelll MDT l.l was used to cllntrol llll' fatigue loadingand

rOl'

data acqllisition(figllrl~:L'i). Figlln~:1.(;S(:h'~!1IaLi·

callyshows the load ("ollll'lli system Ilsed. The sigll,d from IOlulo~lI gw~s tothe l\ITS ControllerI"hl'n'it isfirst1l1l1pljfil~dandd(~l1I()lllll;ltl~(1hyllll~

FI.'Cuback Selector.TIll'St~r\,l)(;olllrull<.'r ("()II1[liLTl!S the f('cdlmck with a(tllli'

posilesignal, t/le slllIHlIaliol1 ofth(~external program (MDT 1.1) andth(~sd up ofdilr(~rcnl [lllr;\1m~tcrsiltlllt~front l:olllrol ]lanel.Ifau t!rror,'xjst.~,an error signal is senl to tlu,S'~rvoValwiwhichopens illlh,~dir<i<:liml and to

(42)

SeaWater link

Ove.head 1-8eim

o

Actultor

Ptltform

Figllrl' :U: Loading arrangement.

(43)

Figure 3.4: MTS Hydraulic controller.

34

(44)

Figure 3.5: The Crack Micro Gauge and MDT testing software screen.

the extent necessary to correct the error.

3.3.2 Data acquisition procedure

Crack gro...th was monitored using an Alternating Current Potential Drop (AepO)technique. The operation is simple. A high frequency AC field is applied to the specimen through two shielded field lead wires positioned at the front surface of the specimen (figure 3.7). Three probe wires are attached to the machined surface. Two of them are positioned across the notch tip forming the active probe pair. The other is placed on one of the machined surfaces which along with one of the active probes lying on the same surface forms the reference probe pair.

(45)

T.

Interlock.

Circuitry

(46)

P,,,b.W"u Specimen notch

1--. ---I

^EO"

Vi= Three channel voltages

,=

R.r.r.nce di.tanc.

a= Crackdepth

Fignre:I.;;ThH'C voltage channels used inAepOmeasurements VI=refer- '·IlCt~.V2=crack 3ud VJ=total voltages

:\7

(47)

Onrrth(~fid'illappli"l,~lllOl"llli.II,lrupillr~'al"IIlC'IW"'l\th,' pOlllc'll, This is IX"CilllSi'lh,' surrar,' k'l'l;lhI)('IW'''~Ithem arb M r,-:<islaw'" to th,'lli,;11 rrcqlll~lICYcurrent.Allshll\\'nillligun' :1.7,V,rPlln':'l'nls 1Ill' IIOlc'ntial,lmp hetwccn thf.' r"ferenre prol)('s andV~r"pn':IClIlsIhe putc'ntiill ,Iwl' lwt\\1,'n the actiwlpruhc~,In n!Js"'ll"C of allY Harktill' following c,'liltinn 1I(,'.ls.

(:\.1)

\Vhen...I'isll\(~rdl'I"l'II""t1i,~lmln',111,1 r isl.I\l~nd sl1rfm'",Iistalll"l'1lt'1,\\""'11 the probe'S Hrros.'! till' nole'h til' (urth'l'1l1'OI)('s), Asitnark l11iti"t,"S O\11tl increases in siz,', so dol'Stil,'slIrra,'"tlislanreIJctwl~nthe;It,ti\'l~"wh,.", '['his gives rise to an im;reilst:tl rl:sislilll('(' and a higlwc pol('nli,,1 limp (V.,) h<,twI'f'1I tile actiwpruhN, IluII"I:\"I'C,IIII' cdl'cmn:vflltagl' V.willhawrt'll1a;!lnl mostlylInrhallgl·d. The n,lalioll ill t"(lllation :1.1 lItlWIwn'l\I'~,

r/V~

=

(211

+

r)/V~

Where,V~and V;~lre lh<~pf'':iI'UI1."....:llli~,1,lilrl:r,'I1"': l...twl"L'Iltill'rdt:r·

Cl1CCilndael In.' proh,' IJltics, l"t",p,,'lln'I}' illitltlislilt'uiu:kdel'tI" 1';<ilIAliulI.~

:1.1and:j,2~i\'C',

timeduring the r,xp<:rilllcnts,

(48)

3.3,3 Crack growth data acquisition

Fi~IJn~ :I.~sdl':lIlalicallyI"f'Jlrt~St'tltsthc sd up used to oMain crack growth ratedl<tlL IlsiugtIl<: i\CPDl(:dllli'jllC. The Um.'Cprobewires arc inputto a 11,'wll'll. l'a..knr,!.~Ci\II11er<vhich switched between different pair or probe v"lll'~':~ (t'I.f(~rt'flCf~,('fi\(·k and tolal,fi~ltre3.7), The output of the scanner, isinpaL lothl)rTad,lIliCT()'.L!,i1n~,:rorlIlodllli~tion.Then thevoltages arc n'nH"'lr~II'rIIII' I('stillgprop;rillll.

Tltl"~' ~I'lsf,f \'ullilg:J'li. I"l'r"l"<'nre, ITilck and totlll, were read each time dataIVa"tl,lIlll<'l"<·d.CI'0tl t.I,e f'Olllpldioli of a desired number or cycles the M1)'1'proj.\rillll slo!lIJl'dl,hl.' ('rdi!:IOilding andheldthe specimenata tensile load ,'nonp;1i10 pn'\'f'lIt thl.' crack slIrraccs from tOllching er.dl other. Then it :<1'111.iIsiJ.(nallo llll' S"i1nlll:l" lo dose' Oflt' ofthe threechannelsarid readinthe

\"011,11-\('.'1'111'\'oflap;c was illPllllo theCrilck f-licro gaugewhich convertedit iUlo11111IIlIplili,',IIJC\"]lllilgC which was thcli input intothePCto be stored ill a dal;\li],~.WlumalllIw tl.l"<:'t)Ch~Ulllc1swereread in,the cyclicloading

TIll'illlpn'SS('llCllm'lItfromtill' CrackMicroGauge was passedthrough

"l'ularil)" ...·\·prsing swilch(1wrnw IJcillll: appliedtothespecimen).Thisgave a,'1';111\'('10kllilnft'lli,'~linoC:illlgChycomparingachannelvoltageunder

!loth l.1u· pularitir's.

(49)

F'igure:I.S:Sc.llemalics of lite dala iKllllisili"n.~ysl'·111 Ils,~l.

40

(50)

3.4 Environmental setup

Alltht~air tl'Sts well:coll(lllcledl1uder Ilormal laboratory conditions at a frc- 'PIt~IICYof :lIlz 1Ll1t!slf/'~~ratioa.II,').Spedal f:nvironmcnts were createdfor nwl"tlsiull r"tiguc l'!l;ls.t\l'titki;d seawall:r l\H(:d in the experiment was pre- p;w,1 iu:mrdillg to thet\ST~lDesignation DI14I·75. A pJcxiglasschamber (Iil-\un: :I.!I,:l.llJ aml:l.J I) was 11SI,t1lo hol.1 tll(l seawater and thesp~cimcn.

TIIt~blilkoftil<' sei\wnl"rW'lSkt:ptilla biggerrest'TvoiTanti~Iowlycirculated lhroll~htil<' tt'Stell'lIlIhel'. The water was cooled insi,le the reservoirby II rl'[ri1\"riltionfoilsY.~!.(:1IIwhich WilS completely co\'ercdbyplastic tubing to Im:\I"I1L any ClllTosion oftil<'acLual copper coil. A temperature IHoDe, ills('rl,~dinsid,' lhl! 1,estchi\ll1b(~rJll'm'ided the feedback to the refrigeration SySll'1l1ellsuring"~1'{',lIl'y ll~~t.ll'Ill!)eratllreof5^Ge.Theseawatersolution was k"pL at apllll'\'{~lllf;l1'lJlll111S.2.;1Spl~rtheASTM:;tanclard~.

J)lIrill~till' (athOilic pmtl'rtiolltt'~t~.the specimen was held at a po·

1.('nLi;dof ·8:10 mV with['j'slwd 1.0 11saLurated calomel electrode(SeE).A

3.5 Experimental Procedure

3.5.1 Specimen preparation

TIll' spl-t:inllmj.~j, ..epar('d for tht'L{~staCL"rding Lo the nature of the test ill\'ulwd. Dl1rillgtIll' rorm,;ioll flltiglle tests specimens were painted with

(51)

Su waler 'ftervoir

Suwata'

Pump

Fi~re3.9: Seawater test setup

Fi~ure3.10: Side view of corrosion fatilue test chamber.

• 2

(52)

Figure 3.11: Top view of corrosion fatigue test chamber.

spray paint. The surface where crack profile is expectedtogrow is stripped of any paint. The notch tip of all the specimens were cleaned so that no metal scrap formed during the machining can plug the crack.

Three

AerD

probes are spot welded to the specimen on the appropriate notch surfaces (Figure 3.7).Thr~thick copper wire stubs were spot welded, twotothe front of the specimen and one to the back. Later the field wires were soldered to the copper wires on the front and the ground wire was soldered to the copper wire on the back.

Once all the necessary wires were filted to the specimen the reference distance (r) was measured using a travelling microscope. Then the specimen was positioned in the testing rig.

13

(53)

3.5.2 Pre~cracking

Ilala for a cr,u:k lip~tm'~intl'llsil)' ran.c,I' 1('\'"lsl",t\\"'~'11lr, to ;,11 :\ll'aJlIl.

To ronfoTllllotlw '\STM slalldaro!:< allSIIl"'il1ll'll~\\·'·"'l'fI·'<"r'lf"k,·,ll"""UI)

lu.ula.titfrcqUt'n,·y uflill:/., TIll' pro'rr,,..k "lI,l-l"ad\\';ISk,'I'1. ,ct '211';{,I"w"l

cr.u·k.~ruwthduring;1.1... illilialpll"~""fth,' .,,·Lnal1.o'St.

Fur 1111'~1·a.lVa1,~r1l'slstllf'~llf','illL"ll.~wI'r,' lln' nark",!illair"I't" ·11"11' aLahighfrt'f)'lI'IH'y r,r 11lIz. TIll' n'S!. lIf tl", !1ff"'T;u'kin,l:; was ,1"1'" illtil<' aclllillll'st I"lVlrUlll1\l'lIt at till' .t<"lual t""t. fn"l'wlwy"r().I!iill~..

3.5.3

Main

test

The actualll~lload \Vas rhos,'n.h~pI'l\,lil1J.("n 111l' l'I"ark J,!;rt!wth 1';11.,' in 1I1l' ('llVironllWl1lil1volv(~d,sothat ':nuugh 11111\11,,·(

"r

datil !,,,illl." ""IIld 1... "I, tainl>U. Itwasattcmpkd nol tuhav,~tl1l' (,""rk,It'ptll da.ta I'"inlst"".-I"~,,.

continued \lntillh.~ ~p"cill1l'1I fail,~,1(,I" rll,g; LI,,: ilir 1.,"SLs) <Jfll,l~<T,,,'k n·;,d,,·,1

solntion.

(54)

• 'I'll<' IOiul wiovdorm. ror load vallie, rrequency and waveform shape,

•C;~lhmlicprolccLiun v(}llill;C,

•Tl~IIIIH'1'iI1,tlrl'and

• pll"rtlu'soillti<lll.

(55)

Chapter 4 RESULTS AND DATA INTERPRETATION

4.1 Data acqusition

beT.r\program. wriU"lI f"rUU~Il1lrposp,~al.ll<'n-d1.1ll" ,laLa1111I[O';,I<-I1[,,'.,-d thea'lCktlt'pth llsingtht~etllli1tiull:1.:1. TI'l"Il.a:; n·nllllllll'llIl(',II,.vtl\l'AS"rM standards, the:;eVt~1lpointslitling methud wa.'! "ppli",lLuLlw ,LIL;l so'ttil.1,- riVt~the~I<'IS d,t/tlNvi\ll1t~.Till: ,!"rlv.:.1dal,;l\\,;~'lI'l"u..l 0111,1"l\l"'rillll'nu,t error points,jflillY, ,1ft)I'xdltde'l. Olin-tll<:datil ar<' lillaliz,·d, LlIO'[,,·sl.Iii.

linesaTC'pilss"d thro1lgh tl'I'1Il to ('vidliall'lll<'mal,pri..1 f"lllIst'lIIts.

(56)

4.2 Computation of crack growth rate and stress intensity factor range

Two sqwrilt<: C"llIllUti,tiollill I'ro(..dllf(·~arc involved to ontain the two pa- r;ollld"">1 n"...·ssal'!' I.. "XI>r<o;" "fal iglll! It>slI't.'sultin auseful andgeometry illd,!pt~lI<l(!nl,IHaUllt'l". O/W uf LlIl-'SI!Ilrofcdurcscalculates the stressintcn- sit)' fll(Olul' nllll-l",ilK.1"1"0111 1Ill.'discrd.! crack lengthlnca.~urcmcl\hilnd lhe l"aoling variabl.·,; for tlU' II.'St sp,·{'illll·n. The other involves computing the iIlSlIlIlL1UI(XIlIS f;ILil!;III'nack gl'()\\,thraLe(dafdN);,fromthe discretemeasure- 11\"111."of till' ('I'm'"11'1l~tll(01,) ;(lHlt!lcelapsed number of cycles(N;).One 'J'i1y lwt·ds tIll' raw erark I,'ugl!, vs. n11mber of cycles data Lo carry out the ahol"- to flllllptl1.il1.ioIIS,

4.2.1 Stress intensity factor range evaluation

TIll' vlldlltioll ill the:<tn'~sinlellsity factor,~l\,ill definl..oU using only the ]lositi\'l' IhHli'lll "fIIH~10i\0I("yelc hecallse of the assumption thatuKis l''1";11toZl'I'Uwhen lilt' crackran-sare closed (dose to the start of comprcs- siVl'load). For th,' WlllpMI Il'l1:<ioll s!H'cirncn,~I<is calculated using the followingrdalioll(,\ST~II·~{i.17-'ST.l!lSL).

I~/').

~l~J_{:'~~\~'!f'l

^x[0.886+4.6-10- 13.320² +1.1.7:!a:¹-:l.nOn·^l]

I'

(4.1)

(57)

Where:.

R =: SpC.:illl"tI lhirklWSS. set' Hgun·:I.:!

IV Spt'cinll'tI lI'itlth,

11.') 4.2.2 Crack growth rate evaluation

The instantaneolls criLck growth rate (dOl/tiN); is wally tJwslllp" of th,' nllck lenglh (il)\'5. dapSI·d ")'<"1,'(Nlcurve ilt. a~h":ll"yd,' N,. i\tl111lll'ricitl tcc1111iqnefilIIbe appliedtu ll!JI."itl th,: slopejl1.~L,·adofadually H'mnlill,l!;

itdUl'illgthe experiment. The jlrt'S.:nl slully uses lIlt'Sl~\"'llpnillt "lIr\',' lil method to

on

Lain theCT;u:kgrowth rate,

4.3 Calibration

Six specimens were tl'slct! in air to ""Iihral,'th,:,\(;1'1)<:ritf:k ,l!;rowlh IW'''' Sllremeflt setup.])urin~till'S!'t",~tsniw);rll'fJlII WilSlH";,sl(rr~1boU1 vislJidly lindby ACPD Lcdllli<ltle. 1'01' t,lll' \·isllillrllt'a.~l1rl~Jlll~lll,Iwadl Blinks WO'f<:

introduced on therractl/rc surf,lt;c al frt:t[llc:ntjJ1ll~rvak'!'ILis wa,11,,,','0111' plishcd by fatigue loading the spts.:irneJl aLtwict~tlw main L<'Stfr!''111I~IlI:Y.'I'll ensure a sharp beach mark, loadrallp;r~wasr'~!lJ(I~11tilhalflll,: mllin tl;sl

18

(58)

vailic. l\ftcr thc test wu Cl'llllplt:tl'fl the specimcn was broken apart and the .:rao:k lcnp;lIls marked I,y the !leach marks were rJlCa5l1rlod \Isin::!; atril~"ClIin~

Cra"kI"II~tllwllllt.'S "I,t.'liut"llusillg both the methods are pJOtleJ ill figure '1.1. Tit" I...,;tlitlillt~oIra\\'11 thl"llllgh all~hedata points yielded the following n:laliun,

T~l=O.!J~J!J:.!{j:lxACPO

+

0.3iO·I'2:1 (403)

Where.T~lalld,\CPO arc the crack depthm~suredby the tr;l.vcllillg Illin"';l:lJpe ami"CPO h'chnitlUe rcspl.'cti\'ely. III light of the above discussioll til<'ACI'D n:u:k,Ieptl. l',tlm::i wI:rc dccmcd to be ilccurate enough and lll"t.'l.i fur allY r.orrLoclioll\\'a.~found uunecessary.

4.4 Results

'I'll<' rati?,ll<' nack gruwlll data til~'sN) for all lhe tcsts are showll ill figurc ,I.:!throl1l);h·I.!J.

'I'll<' crack growth rate .!ala arc pfl'Seutedgraphieall~'as log. log plots of na"k growth r<llt: (clafliN) arul5lress intensity factor range~Kin fi,gull'4.10 thruu.~h

-un.

Individual tL'StS arc prl":Stmtcd in seMter plots. Then the until (rom l('sts carrit·d out at a parlindar cnviro1llllcntMegrollpL',j togethl!r ill OIl<'sill.~I.,plol. This gaVt' a ,·lran....' to.~l·,11I1all·thc\'alidity of the test results.

As;LIlt';,;t.itl'Jl.I"ast sllll:nl' n'sTt'ssiun curvc wn.s fiued through a gro'll> of

'9

(59)

10 15 20 ACrOCri\ck l.cnJ!;lh

'1U'1-~-~"-_._~---;

:!O~

i

n\ JI Cr:lck I~L l~ngth

'I

JOl

"r i I

o'---~-~--~-~

I

o

Figure,\.I:ACPD vs. TM crackdl~plh.

,jO

(60)

daLa. Tlll~form ofll".~equation in Pilris' law (UJ63) was used in the least squilre lJrllgrillil,

Wht:re, C andIIIarc callcd the material constants.

(-!AI

Duetllthc nunlineMityor thedala points (in log-log sCille) ror the corro- siollfatiWH! t.~ts,morc than one such curves were fiUed to dost'ly approxi·

maLt~tli,: da./dN vs.6,1<plot. Table 4.1 lists thc material constants derived fmllltIlt: !Jest tit,~urwsall,lthe(orr~spolLdilJgrange of~I\value:;.

L.:::':.!\

RanL;<' C M Air

!(j.:!:l:lto ·17.i:ll 2.26'125ge-12 3.:i22270 Scawa1er

:!0.12Ijto:!7.00J ·1.20203ge-13 4.003989 :n.0031035.7i6 1.841166e-IO 2.152813 :15.776to48A7:,) 3.143066e-08 0.713390 '18.4i5to61.137 1.720893c-(O 2.062689 Cathodic Prot,xtion

:.10.~:l!)to :.11.841 1.::l10432e-23 11.819560 :.1l.S·l! l025.114 :l.510724c-06 -1.138005 :!5.1l·1 to :17.5:12 8.7:17302e-[3 3.595440

Table .1.1:~latcrialConstants

;i1

(61)

4.4.1 Air tests

Figure ·1.10. ·1.11iI/ld·1 :.:~htJl\'rill' l'r'lrk groll'th ral,',Iill.af<lr1./1<'I.ltm' outorthefi\"<~ lL'lit~l"Imd\l{'\l,dill air. Finally they ItTt' l"omhilWd in 1IIll' pint (figure '1.13) nlulit~ingll'bl'st firlinei~ llil~s{'dthrough 1.11('111.

The firlit spc<:il11cn (AI) waslC'~t.<,<1at<I.higlu'f IOlldT'II1/;e.In .ml,-r10 obtain more numherordata[l<lilll.~itlI"a.~dN:id<'11 toIm\'<~rI.h,' IUilt!rilllll.I' ,HId dday the r,li!ure of Ihe sp1'61l1('11. However, rrol1l Lhe l'IJlIlhilll,d plotit

int,ensityrillw'ofII)~'ll'i1Jtll.Tilt,n)lllhilwtlplot(li~l1fl!·1.I:l)shuws a\'I'r~' goo{llIgrC{'melltlwl,wet'llllll.~ l.hr(~'l,'stI'{'slilis slLOwillg til<'~oUlIn'[ll'alal,ility of the experimcllt.

4,4.2 Free corrosion tests

'1.15, '1.16<tilt!fiuil!ly iltIJt:Olnhiut:d 1110t jig-lire·1.li. TIll:jjr.~t ~1'1'l~jlrlf'tJ (51) wastested at a highcrrall~c(.6.1'=25.fi28 kN)I:Ompafcd to the oLlu!r two(fiP=2:1.298 kN).'\.~Sl:':lIill ligure .1.1,1,specilllllll IO:f1u:kg-rowth rate showcU a plateau lwbllVionrh~'owit61\ valm: ofa:IMP"Jm. 1I1:yolifl tllM the crackgl'Owtl'hclmvi()r\\'iL~ idt~lllieidLa tlw ol.her twoSP(!r.illll:IIS. III the sllrcrimpo..;cd plut (ligllf(:·l.1i/(ollr IJf:slfiLIiJl(~swew drllwn to lwsl rcprcsent thedilL,!Sl~t.DlIrin,!!;till'IJesllitpron."s onlytIll:portioll"fHI<,

.j:!

(62)

spt1:inWll I da.ta that is similar to tbe ollicr two is considered.

4.4.3 Cathodic protection tests

Fi!;llrl··!.IK ;III'} ,!.I!/ display 111<' dnlll for the twosJlf:cimcns tested in seawater wilhI'nL!lodit: proLCI·l.itJll (·,s:HJ mY"SeE),For mosL part the cr,'lck growth ratl' was illll.·I\Lit;al tolll(~air values. rlclow a~I\value of 25MPa..;m bolh lllf~sp"fil1l('IIS showed "hrier I'Niod ofcrack growth rale higher than in air.

Forfirst sp"('illwllCl.Lhl'f(' isasharp decreasein crack growth rate aftertl.K

=

'llil\II'IIJlIl Tllis forrespntuls 10 "loading system failure lind shut down of 1.11l'l.losl, for fi-ihU\tts. lIo\\,(·\·<T.till'crack growth ratc again recovered to lhilloftIll:,~pl'('ittll'll

<::!

1](':11' 11.ll\ valul'of30MPa.;m.F'igltre4.20shows Lim l'ol1lbin",1 plol of lhe fatllodic [lrolcc\.ioll tests.

(63)

:!l)---~~....:-_--_..• _---_.-

1110000 ~oouuo nuuuo ~'IOUOtl :!liIlIWHJ

(64)

fill

--·---,-··r---,---,---.--.--,----.---.-,

lif)-

·11i

·Ill-

:lO

" --'---,-t-,-,-L-,-'-"-".,L-,--,,-L-,.,.,L-,-:':-c-,,.,L-,-.,-,-J o fiOIJOO lllOOOOI500002UOllO025IJOOO31JIJOOO3500IJ0400000'151JIJOO51JIJOOO

l:ydcs(N) Figure 4.3: Crack growth curve for: air 2

55

(65)

2" - - l - - L . - _ L _ o 50000 100000 150000 20000025ססoo:lOOOO(13!.oooo-10ססoo-I'.ocxwlrMIlIIlllO

Cydcs(N) Figure'iA: Crack growlh curVlJ ror: air;\

.\6

(66)

~S.~aw"Lcr:I

;m -

:\~O'·OOC'-::::20U'::OU'-::::300'::OO

⁴⁰⁰⁰⁰ ⁵⁰⁰⁰⁰ ⁶⁰⁰⁰⁰ ⁷⁰⁰⁰⁰ ⁸⁰⁰⁰⁰ ^{lOOOO

Cycles (N)

Figure ,1.5: Crack growth curve for: sea water 1.

57

(67)

60

55

50

~ 15

} "

a "

JO

25 ._L __

20000 .\0000 60000 8ODOO 100000 120000 140000 lIiOOUCI 111111))/1

CydCll(N}

Figure: 4.6: Crack growth curve for:Sl:,lwalc:r'I..

.58

(68)

44 - - ..---,---,--~--___r--~--_,

40-

:111 orS<~"walct::1

:u;-

:lU-

28-

211.---~.-'-c_ _,,"cc--__:c_L,_-_c:_:'cc:_-~::c:_-,.,.J

20000 'WOOO (;0000 80000 100000 120000 140000

<;ydcs(N)

Figure ·1.7: Crack growth curve ror: sea water 3.

(69)

50

J

25 _....1...--... ..1 1.. __. I

II :WOOO 40000 llOmO !lUUOO lUOOOO12(11111) 14011111l Ili/J(JUIJ

Cyd~.,.(N)

Figure 4.8: Crack growth curve for: (atho<l!cpmkdil'lII.

fiO

(70)

111 --

-,--,,----.---,r----.----r--..,

111 - .e-CUboJic prolec:lion: 2

3·1-

:1:1-

:w·

'UI'- I !

II 10l~1O '10000 6OUOO 80000 1ססoo0 120000 I'HlOOD

Cyck",(NI

Figllrt~.1.\);Crack growth curve for: cathodic protection 2.

61

(71)

J I

-.r ..

I -

IOl~-'~~~- .. '-=:: ' '~

[- I

I

IO-'~ i

~.

r

!

"L_

^10'

(72)

FigUrl'.1.\1;Ai;lest: specimen 2.

63

(73)

--:---·1

; -~Air::~-.

···-i--·-.·-

. .

^-

^.

...l... :

I

10-1 L - __ L .. _I__ I _l--'~:J.'

10· II)~

Q,/\/MP'·Vm )

Figun:'1.I~:Air lr.sl:Spl:dllll:n:I.

&1

(74)

10-¹

IO-~OL,_ _----'_---i_'---l.-Ll--LLJlO'

65

(75)

10-'

f-._-_._... .,.-/

t 1

e - - - l I J I J..l IJ1

10-101- " - --_. -- .. IIJ~

nK

(~II'aJIrIJ

F'iguw'I.l~:Sea waterfre(~mrrosiou:spf:.:illll:1II.

G6

(76)

Figllrt' t15: ::lei! walerfrt!C corrosion: specimen 2.

(77)

Figuw,I.lG: Sell waterrr(~l!I:orfusioll: Spl)f;ilr1l:lI:1.

68

(78)

10-⁷

lO-;~O·-_--JL.---'-.L--'-.Ll...LLJIO' .6.K( MPa

v"")

Figm~'tli:St-'1lwatcr t<..~l:Best fit lines,

69

(79)

10-11 , - - - , - - , - - - , -

j---.J..._L~~_I_;._i_1

IIJ~

Figure 4.18: Cathodica.llyprotr:dl~ll: ~pf.'cirw:1II.

70

(80)

:~:~~~~=l:!mg

__ ;L.._

:~.:._U.! ~

~j!1

.-.----,-,.-~...____;_-:~-"f"-:

---H--~.--+-+-_:_++-

'

~~~=~=1;-t=:--t= i i-t~

,: i : f ~ ~ I :

!

...- ----"/"-r----r"'---r-T-r"T"TI

! : • CaLooJic _{Prbl~I~: ~} ! -=-=:~:;-~t·il·-·l·_·t

. . i ! '

! .

Figure <1.19: Calhodically protected:gpccimcn 2.

71

(81)

/1

/1:

10' L\K(MPaJm)

72

(82)

4.5 Discussion of the results

TIIf:n·.~IlILs[milltlu'!.·Xjlt-'I'irnellLsare presented in diagrams where the crack

!:l'flwlh fnL" dll/lIN arc pilltled as[lfuuction or the stress intensity factorllK.

The faligw! crack l;mwlIJ rate in air was~cleclcdas a base for comparing the ],,!haviullr

or

thehi~SC:U1etal ill other environments. The results for the tests ,",,,ulm:!.,:.!ill ;lir nn:W't)lIIWdtogctllf:rlind presented in figure <\.13. The data I'"illt.~an'r~,il'lyw,·lIwpr.·,;o·lItcd II}" the bestfitstraight line,

(...,) 11'111'1"1', fl;J/IINis illm/erell'alld~l\is in MPa.jrn.

The;\IJ;IWll\t'ULhct\\'('(~nthe tllfCCIc~~resulLs arefairlygood with a little

Ili,\lPaylJ"

'I'hl' ;,.II'II'IIC\' of seilw"l..'r 1\'1\,'( il1\·e:;lignted and the results compared wilh I.he air I."st r<':IlIlts nrc pres"ulcd ill figure 4.17. All thete~tswere conducted at a [ll,ltl ratiu of 0.0:) aridilla fretllll'I1CY of O.lfi7Hz. The pH of the seawater

\I'm;[,"pt,II,~.:!.Vt'rrliUIl'dHlngeillpll occurred duringthe course of the

"Xpcrilllt'lll..Th.: 1,'mlll'l·'Il,lIr.: of the seawater was kept constant at SoC.

,\s ,'anIll'S,'t'll[rulllligllrc ·1. [7,the ,,[feet of seawater on the fatigue crack l!,rtlwth l'a1o:is uot. IIniforltlU\'l'l'lherange of.6.1\ At smaller crack depths (Iow,'r ,),1": )tilt'nilrkgl"Ol\'lhral,' is lo\\'er than in air. Ho\\'ever, between a

(83)

l:::./\\'alue of 20 to.IO~IPavllll'ri\d,gr,)\\·tllcxn...·ll t.lleair10..'\'1'1.Atilhi~ll1'r levelof6.1-\theilltll1l'nCI.-' of SI'i\Wat('I'1I'a.~negligibl('. This is llIailily 1ll't'ilnSt.

of the domillallce of high~l\owr allY dll'lllic.\1 activity.Similarnhsl'rvatiulls have beer}l't']JurtcdbySclloltt:alld 'Vildst:llt (1!ISI) alltli'llsl.l'll,d. ,,1 (l!l.'il).

Thedfcct of c<llhul/ic prol''CliollIII the base nwtal WilSill\,,'1tti/!;I~It... 1 by proteding tlw llpedllH'n wilh a,'n1.!lndic polt'lltial nf ·1'\:UlllIV (S(:E). Th,·

r0;ulLs obtainedarlOpn·st·lltt'l.l illI1gnrt'S·I.IS"nd·1.1~1.AsSe'I.'1lillli~lIrt·s ,1.18 <lnd '1.19 the (rack growth 1',,1(' is ahnolSlid':Il1.i'~all()till: "ir \';dw'll.I\l low~l\(below:~OMllav'l1l), the riltt' ishe·lowLII'lair ,',,In'!s.l'~lr.~ sli~htly higher.lK (hdw("'11:!O.:!,'iMPily'lll ) 1.l1I'I'1: is alJri"f riSl:ur tlwrilte I,,{umit falls hack(.0Ihl'airh'l'd. This.~II,ld'·lIris.. 11lI11 r;dlorthl~enu:k~r(JwLhmho w:u;obs"!rvislin hoth lhl:sPI'(itll!'ll.~LI'Sl",I, II (',til1,1'SI~'11lISa,~UlallI,lall'all

Tlie plot for theSpt'dlllCIII(li~llre'1.18) shows as\ldcli~1Idrop III the:

crackgrowth rale I/"luc iU'mUIl!l:::.l(o(:!!jMP".;m hdllrr. it WillI'S hark to lheairIc\'t~1\wy '1uiddy. This ahrupl dl'pal'llirefrol11thl'st.I·lulysl;\t.,~1/,11111' coincideswithitloadin~systcm failur(' illldsto(JllilgeOfUII~ e~xp,:rilllellLfill',~i"

to sel/cn houts,Therc Illight have lm:1I a ve:l'y small load b:epin/\ t111: rral'k faceopen while theanodicdissolution l,luuled thec:rark Lip.i'~SOOIi;\~till:

experimenlstartedlilecracktipregiL;IJ(x!nssho1rpllc.~sa'1IJcrit/:kJ{rowLIl ratl:

vallie increased to the steady~ti\te~leI/d. Prom ihf:re~mllsohlilillisl fmm the

1·'

(84)

.X:llJIIIV (SCE) dimilll.l':Sillly drl.'Cl "r sc;, willt:r UI1 llll' ralisuc crack growlil

"fll ... lJilSI·lllf'llll.

",uls. The r..'S"ll~arl: "'pUrkll illApPcll~lixA.

i5